Principal motions in looms are all correlated to the angle of rotation of the prime mover which drives the loom. The timing with which main and auxiliary nozzles eject weft inserting jets is controlled generally by a mechanical cam. The cam is driven by the main shaft or spindle of the loom in synchronism with the operation of the loom.
The RPM of the rotating parts of the loom does not reach a normal RPM during transient operating conditions, such as when the loom is just started to operate. As a result, a jet of weft carrying fluid is ejected at the start of the loom for a longer interval of time than the time interval for which the jet is ejected under normal operating conditions notwithstanding that the ejection starts at a normal angle. This problem arises out of the fact that under such a transient condition, it takes a long period of time for the rotating parts to make one complete revolution and hence to move between certain angles. Therefore, the jet of fluid tends to be ejected at a greater rate than necessary during the initial period of operation of the loom. This sometimes causes the weft yarn as it is being inserted to be broken by the jet, and to be bent at its distal end due to any difference between the timings of operation of the main and auxiliary nozzles, resulting in unstable weft inserting operation.
To prevent the foregoing shortcomings, a jet loom disclosed in Japanese Patent Publication No. 57-38699 (corresponds to British Pat. No. 1,596,964) controls the flow rate of a weft inserting fluid in relation to the RPM of the loom so that the flow rate will proportionally be small at low RPMs. With the disclosed jet loom, however, the weft insertion remains still unstable as the weft yarn runs under different conditions at the start of the loom than those under which the weft yarn is inserted while the loom operates normally.
The weft yarn as inserted through a warp shed is detected by a weft feeler at the edge of the fabric being woven which is remote from the ejection nozzles. The loom has a control unit responsive to an output signal from the weft feeler for sequentially controlling the operation of the loom while monitoring the condition of weft insertion. Therefore, the time at which the weft yarn reaches the fabric edge remote from the nozzles serves to provide a critical timing for the control of the loom. If a jet of weft inserting fluid is ejected under the transient conditions for a prescribed period of time starting from the same angle as that in the normal condition, then the rate of flow of the fluid is equalized to that in the normal conditions, but the weft yarn reaches the edge of a fabric being woven remotely from the nozzles at a time faster than that in the normal condition. As a consequence, the loom cannot be properly controlled.